In general, it is known that the surface properties of solid are improved by forming a thin film on the surface of solid material in plasma, or by injecting ions into the surface. A film formed by use of plasma including metal ions and nonmetal ions strengthens abrasion resistance and corrosion resistance of the solid surface, so that said film is understood to be useful as an overcoat, an optical film and a transparent electro-conductive film. In particular, a carbon film formed by using of carbon plasma has the high utility value as a diamond-like carbon film (called DLC film) being a mixed crystal consisting of diamond structure and graphite structure.
As a method to generate the plasma including the metal ions and the nonmetal ions, there is a vacuum arc plasma method. The vacuum arc plasma is generated by an arc discharge occurring between cathode and anode, and is the plasma formed by cathode evaporation substance of the cathode material that evaporates from a cathode point existing on the cathode surface. In addition, when reactivity gas or/and non-active gas (rare gas etc.) are introduced as atmosphere gas, the reactivity gas or/and non-activity gas also are ionized at the same time. By performing film formation and ion injection to solid surface using such plasma, the surface treatment can be done.
In a plasma generating portion of the plasma generator, under the state that an electric current is flowing between the cathode and the trigger electrode which come in contact mutually, the plasma generation is induced by separating the cathode and the trigger electrode. In the plasma generating portion, vacuum arc plasma constituent particles such as cathode material ions, electrons and cathode material neutral particles are emitted by vacuum arc discharge, and at the same time, cathode material fine particles (called droplets as follows) in size from submicrons to several hundred microns (0.01-1000 μm) are emitted, too.
One part of the present inventors proposes a plasma processing method using the vacuum arc discharge described above. This method is published as Japanese Patent Laid-Open No. 2002-8893 (Patent Document 1) and is shown in FIG. 7. FIG. 7 is a schematic diagram of the plasma generating portion 102 in a conventional plasma generator. In the plasma generating portion 102 arranged in a vacuum chamber, as the preceding step for generating the plasma 108 between cathode 104 and anode 140, an electric spark is occurred by trigger electrode 106 and then the generation of plasma 108 is induced. It is not illustrated, but a driving device which can drive up and down is provided near the trigger electrode 106, so that the tip of trigger electrode 106 comes into contact with cathode surface 104a by the driving device and the voltage is applied to a contact point by arc power supply 122. An electric current concentrates on this contact point, and when the trigger electrode 106 is separated from the cathode surface 104a, the electric spark occurs and the generation of plasma 108 is induced.
However, an emission bore 104b is formed on the cathode surface 104a when the plasma 108 occurred. Namely, the plasma generating substance forming cathode material in this emission bore 104b is emitted as the plasma constituent particles and droplets 118. When the trigger electrode 106 approaches said emission bore 104b, the generation of plasma 108 cannot be induced because a desirable contact state is not formed between the trigger electrode 106 and the cathode 104.
As a means for solving the problem of the emission bore formation on said cathode material surface, in an electrode constitution disclosed by Japanese Patent Laid-Open No. 2001-192815 (Patent Document 2), an abrasion device reproducing the cathode material surface to the shape of a plane is disposed with a driving device to turn the cathode material. It is not illustrated, but even when the plasma generator of FIG. 7 is intermittently operated for a long time, a cathode turning mechanism device rotating said cathode 104 in which the rotational axis of cathode 104 is a center line perpendicular to a bottom of the plasma generating portion needs to be disposed with an abrasion device abrading the cathode surface by a grinder. That is to say, in the case generating the next plasma after the formation of emission bore 104b, in order to assure a desirable contact between the trigger electrode 106 and the cathode surface 104a, the contacting point of the trigger electrode 106 with the cathode 104 is sequentially shifted by turning the cathode 104 using said cathode turn mechanism device.
Furthermore, since a lot of emission bores 104b are formed on the cathode surface 104a by repeating the vacuum arc discharge, it is necessary for the cathode surface 104a to be ground with the abrasion device by stopping the intermittent generation of plasma 108 at one time. Since the flat cathode surface 104a is formed newly by this abrasion process, the trigger electrode 106 and the flat cathode surface 104a always comprise a good contact state and the generation of plasma 108 can be induced again. However, while the cathode surface 104a is being ground, the intermittent generation of plasma 108 is stopped at one time and therefore it made reduce the operation efficiency of the plasma generator. In addition, since mine dusts occurring in the abrasion process pollute the vacuum chamber and deteriorate the quality of formed film, collection of the mine dust and cleaning of the vacuum chamber were necessary.
Conventional trigger electrode 106 shown in FIG. 7 is arranged in proximity to the cathode surface 104a. In this case, deterioration of the trigger electrode 106 becomes early because it is exposed to plasma 108 and heated. In addition, the membrane that accumulated to the trigger electrode 106 exfoliates and is mixed to plasma 108 as contamination matter so that the problems to obstruct advance of plasma 108 is brought about. In order to solve these problems, a driving mechanism moving the trigger electrode to a plasma collision avoiding position is provided with the plasma generator disclosed by U.S. Pat. No. 6,319,369B1 (Patent Document 3), and the plasma generating portion of patent document 3 is shown in FIG. 8.
FIG. 8 is a schematic diagram of the plasma generating portion in a conventional plasma generator. A cathode cooling member 164 with inflow portion 161 and outflow portion 162 of a coolant is installed by flanges 165, 168, and the cathode 104 is fixed to the upper part of said cathode cooling member 164 by a fixing member 169. A magnetic coil 168 installed around the anode 140 makes converge plasma, and an induction magnetic field leading the plasma to a plasma drawing portion (not shown, upper direction of drawing) is formed. In addition, baffle plates 173 suppressing the progress of droplets reflected by a side wall, introduction portion 134 of reaction gas and observation window 166 are arranged.
In FIG. 8, the trigger electrode 106 and the surface of cathode 104 are in a contact state. When the trigger electrode 106 is separated from the cathode surface due to the driving mechanism in such a contact state, a cathode point is formed at the position where tip end portion 106a of the trigger electrode 106 and the cathode surface were contacting each other, so that arc plasma is formed in the direction from this cathode point to anode 140. In addition, said driving mechanism is controlled by means of controller 167. In order to prevent deterioration of trigger electrode 106 based on heating and mixture of contamination to the plasma and not to obstruct advance of said plasma, the trigger electrode 106 is stored at the plasma collision avoiding position (position of trigger electrode shown in broken line). However, since the emission bore of plasma is formed on the surface of cathode 104 in FIG. 8, it is necessary to install the trigger control device and the abrasion device as well as FIG. 7, and is necessary to wash the vacuum chamber frequently.
[Patent Document 1] Japanese Patent Laid-Open No. 2002-8893
[Patent Document 2] Japanese Patent Laid-Open No. 2001-192815
[Patent Document 3] U.S. Pat. No. 6,319,369